Three field experiments to investigate the partial nutrient balance (N and K) in cassava/soybean intercrop system were conducted at University of Nigeria, Nsukka in a derived Savannah location of South Eastern Nigeria. Effect of fertilizer rate, residual fertilizer and soybean residue management on growth and yield of soybean and cassava and soil nutrient reserve were studied. Soybean growth and yield were significantly affected by their varieties, fertilizer rate and cropping system but not by cassava variety. Application of fertilizer at 50 Kg ha-1 fertilizer rate gave the highest soybean total dry matter (898.24 Kg ha-1), highest grain yield (156.91 Kg ha-1) and highest fresh cassava tuber yield (30.7 t ha-1) at 12 months after planting (MAP). Intercropping cassava variety (NR 8230) with medium maturing variety of soybean, (TGX 1894-3E) gave the highest grain yield of soybean (133.60 Kg ha-1), highest fresh tuber yield of cassava (30.8 t.ha-1) at 12 MAP, highest N and K balances (+44.06 and +72.70 Kg.ha-1), highest LER of 2.71 and ATER of 2.10 at 50 Kg K.ha-1 fertilizer rate. The effect of residual fertilizer on soybean plant height, litter weight and shoot dry weight was highest at 45 Kg N.ha-1 and 50 Kg K.ha-1 fertilizer rate, while soybean grain yield of 204.1 Kg.ha-1 was highest at residual 50 Kg K.ha-1 fertilizer rate. Also, cassava leaf-N (4.05%) and cassava stem-K (2.8%) were highest at residual 45 Kg N.ha-1and 50 Kg K.ha-1, while cassava tuber yield at 12 MAP was highest (24.27 t.ha-1) at residual. 50 Kg K.ha-1 Soil nitrogen increased from 0.04%N to 0.34%N and soil- pH from 4.2 to 6.6 at sole soybean. In sole cassava, soil-N decreased from 0.04% initial soil-N to 0.03%N but soil-N under 3 year natural fallow was unchanged. Residual fertilizer affected nitrogen balance of sole cassava which was negative, while intercrop had positive nitrogen balance at all residual fertilizer rates. Potassium balance was positive with intercrop at all residual fertilizer rates except at no fertilizer. Incorporation of soybean residue into cassava/soybean intercrop system increased cassava tuber yield and soil nutrient status and resulted in positive N and K balances. Analysing of nutrient stock: balance (NSB) ratio revealed that fertilizer rate of 50 Kg K ha-1 in the first year of cassava/soybean intercrop followed with incorporation of residue in the subsequent year (without further application of fertilizer) gave the highest dynamic reserve of 310.6 Kg N ha-1 and 311.64 Kg K ha-1. Under sole soybean, production can be sustained for 15 years (NSB ratio 14.58), while under intercropped system production can be sustained for 9 years for nitrogen and 9 years for potassium (N balance of +62.64 Kg ha-1 and K balance of +87.66 Kg ha-1). Sole cassava without fertilization and no residue incorporation can be sustained for one year with NSB: ratio of 0.88.
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